Dhaka has once again topped the list of world cities with worst air quality. With an air quality index (AQI) score of 251 at 8:53am today, Dhaka ranked first in the list of cities with worst air.

An AQI between 201 and 300 is said to be "very unhealthy", while a reading of 301 to 400 is considered "hazardous", posing serious health risks to residents.

India's Kolkata and Vietnam's Hanoi occupied the second and third spots in the list, with an AQI score of 201 and 192, respectively.

An AQI between 101 and 200 is considered "unhealthy", particularly for sensitive groups while a reading of 301 to 400 is considered "hazardous", posing serious health risks to residents.

In Bangladesh, the AQI is based on five criteria pollutants -- Particulate Matter (PM10 and PM2.5), NO2, CO, SO2 and Ozone.

Dhaka has long been grappling with air pollution issues. Its air quality usually turns unhealthy in winter and improves during the monsoon.

Air pollution consistently ranks among the top risk factors for death and disability worldwide. Breathing polluted air has long been recognised as increasing a person's chances of developing a heart disease, chronic respiratory diseases, lung infections and cancer, according to several studies.

As per the World Health Organization (WHO), air pollution kills an estimated seven million people worldwide every year, largely as a result of increased mortality from stroke, heart disease, chronic obstructive pulmonary disease, lung cancer and acute respiratory infections.